The goal of our studies is to understand biochemical pathways and to identify key proteins (enzymes) that are involved in the activation and effector phases of important immune responses, such as T-cell-mediated cytotoxicity (toward virus-infected cells and toward tumor cells) and lymphokine secretion. It is expected, that this knowledge will enable us to selectively manipulate different functional responses of T-cells with specifically designed immunomodulators. We identified a novel trans-cell ecto-phosphorylation/dephosphorylation system in lymphocytes, which supports a newly formulated general model of cell-cell interactions in the development and effector functions of lymphocytes. According to this model, receptor-ligand or cell-cell contact protein interactions are regulated by the state of phosphorylation of extracellular domains and by signal-transducing ATPo- receptors. We demonstrated the presence of, characterized, and purified a highly active and extracellularly located Casein II-like protein kinases (ectokinase) which we implicated in T-cell function. We also demonstrated the presence of a PPI class ecto-protein phosphatase. In addition, we cloned the cDNA coding for the T-cell's ATPo-receptor and obtained antibody to the extracellular domain of this receptor for future studies. We found that substrates of ecto-phosphorylation on the lymphocyte surface are either functionally important surface proteins themselves or are associated with them. The cDNA cloning of these ecto-enzymes may have immediate immunopharmacological implications by allowing the targeting of novel surface molecules. We continued studies of the role of cAMP-dependent protein kinase (PKA) by obtaining stably transfected cell lines and transgenic mice with a dominant-negative R1alpha subunit mutation.